Coating a fluid onto a web of material is well known. Extrusion of material so as to form films is also known. Such coating and extruding can often be conveniently done using a die having a cavity communicating with an applicator slot. Liquid under pressure is introduced into the cavity, and is then extruded out of the applicator slot as a film or onto a desired substrate or as a film.
Depending on the exact result desired and circumstances surrounding the coating or extrusion, various aids and orientations of the die may be utilized. For many types of coating or extruding, it is convenient to orient the die so that the applicator slot is disposed towards the top of the die. One reason for orienting the die in this fashion is that any air (or other gas) introduced into the die during operation, or air remaining within the die after the initial introduction of liquid into the cavity of the die tends to bubble upwards towards the applicator slot. This allows air in the die cavity to be eliminated. This is desirable in that residual gas within the coating or extrusion die, acts to reduce the response time to start and stop the emission of liquid through the applicator slot. This unresponsiveness is due to the compressibility of gas, versus a cavity completely filled with incompressible (or substantially less compressible) fluid.
For some extrusion or coating applications, however, it is desirable to dispose the applicator slot towards the bottom of the die (i.e., orient the die such that the applicator slot is disposed downward). This problem is particularly common when the liquid is to be coated onto a substrate in discrete, separated patches, when die responsiveness to starting and stopping of coating is particularly important. The problem of removing residual gas from the coating die when the applicator slot is disposed towards the bottom of the die has been considered by the art. It is known, for example, that when patch coating discrete articles a bleed valve can be provided for the die chamber so that any air coming into the applicator die is bled off through the air bleed valve.
However, pockets of gas can still occur in the die cavity, which are not eliminated by the bleed valve. These pockets of gas can especially occur when the die is particularly wide. Thus, the art still requires some way to assure removal of residual gas that is more generally applicable to varied die geometries with the die oriented in various directions.